Rapid growth of the wireless mobile communication market created demands for various multimedia services in a radio environment. For massive transmit data at high data rate using limited resources to provide the multimedia services, researches are conducted on a multi-antenna system, for example, a Multiple-Input Multiple-Output (MIMO) system.
Compared to a single-antenna system, the multi-antenna system can increase a transmission reliability and a data rate by transmitting data over independent channels per antenna, without additional frequency or additional transmit power allocation.
When the multi-antenna system adopts a spatial multiplexing scheme, a transmitter transmits data through a plurality of transmit streams at the same time. In so doing, the transmit streams sent from the transmitter may cause interference and degrade the performance of a receiver.
The receiver of the multi-antenna system mitigates the interference by nulling the interference signals in the channel direction of the stream to detect based on Minimum Mean Squared Error (MMSE).
Disadvantageously, as receiving the signal based on the MMSE, the reception performance of the receiver is degraded further than a receiver based on Maximum Likelihood (ML). In more detail, when receiving signals based on the MMSE, the receiver generates a weight vector for the nulling by treating other signals than the desired signal as the interference. Accordingly, for a plurality of desired signals, the receiver generates the weight vector for the nulling without considering the desired signals and thus the nulling effect is diminished.
Further, the receiver based on the MMSE cannot utilize all of gains in the multi-antenna joint detection, unlike the receiver based on the ML.
When receiving signals based on the ML, the receiver does not consider the interference and thus the interference degrades the reception performance. Herein, the receiver for receiving the signal based on the ML is constructed as shown in FIG. 1.
FIG. 1 is a block diagram of a receiver in multi-antenna system.
The receiver of FIG. 1 includes a plurality of (NR-ary) receive antennas, a front-end processor 101, an ML detector 103, a Log Likelihood Ratio (LLR) generator 105, a decoder 107, and a channel estimator 109.
The front-end processor 101 converts Radio Frequency (RF) signals received via the receive antennas to baseband signals.
The ML detector 103 selects and outputs a symbol having the shortest Euclidean distance using all of candidate symbols applicable to the receive signals output from the front-end processor 101. When selecting the symbol of the shortest Euclidean distance, the ML detector 103 takes into account only its desired signal, not considering interference signals.
The LLR generator 105 generates and outputs a soft decision value (=LLR) of the symbol output from the ML detector 103.
The decoder 107 decodes the LLR value output from the LLR generator 105.
As stated above, the ML based receiver utilizes the separate LLR generator 105 to soft-decode the candidate symbol selected at the ML detector 103. However, since the ML detector 103 selects the candidate symbol without considering the interference signals, the interference signals are recognized as noise and the selected candidate symbol includes the considerable noise. As a result, the LLR generator 105 generates the incorrect LLR value because of the noise in the candidate symbol output from the ML detector, to thus drastically degrade the reception performance.